Highly sensitive photoacoustic gas sensor based on multiple reflections on the cell wall

Abstract

A highly sensitive and miniature photoacoustic (PA) sensor based on multiple reflections on the inner cell wall is presented for trace gas detection. The collimated laser light is obliquely incident from the side wall of the cylindrical PA cell. Multiple reflections occur on the inner surface of the cell wall to increase the length of the absorption path. The PA pressure signal can thus be enhanced. The PA signal is measured by the second harmonic wavelength modulation spectrum (2f-WMS) technology to eliminate interference from the fundamental frequency signal generated by the wavelength-independent absorption of the cell wall. The internal volume of the PA cell is only 848 μL. To further minimize the PA sensor, a micro-electro-mechanical system (MEMS) microphone is used as the acoustic detector. In order to evaluate the performance of the designed sensor, trace C2H2 is detected as the target gas at the wavelength of 1531.588 nm. Experimental results show that the amplitude of the PA pressure signal using the multi-pass cell is 6.4 times that of a single-pass cell. The detection limit is achieved to be 31 ppb for an averaging time of 400 s. Furthermore, the normalized noise equivalent absorption (NNEA) coefficient is calculated as 1.1×10-8 cm−1 W Hz−1/2.